This invention relates generally to a low resistant switch which buffers output circuitry coupled thereto from extreme voltage transients that may occur at the input of the switch. More particularly, the present invention relates to a three terminal integrated low resistance switch circuit suitable for use in an automotive application to protect load electronics coupled therewith from voltage transient that may occur at the input thereof which otherwise could damage or destroy the load electronics.
Any electronic circuit utilized in an automotive system may be subjected to abnormal transients that occur on the battery bus. For example, the nominal battery voltage appearing on the battery bus is +12 volts but under jump start conditions, when two batteries are placed in series, +24 volts may be supplied to the electronics coupled to the battery bus. Another abnormal condition that can occur results from the battery terminals being reversed. Under this condition upwards to -13 volts may be present on the battery bus.
Still another abnormal condition specified by the automotive industry is referred to as voltage load dump wherein it may be possible to have 60 volts occurring on the battery bus. One other condition specified by most automotive manufactures is that all electronics must be able to withstand .+-.125 volts inductive transients on the battery bus.
Most semiconductor manufacturing processes used today to fabricate monolithic integrated circuits for use in automotive systems can withstand 24 volts applied to the integrated chip. However, in almost all cases, present day solid state electronics cannot stand the 60 volt load dump condition due to breakdown problems as well as power dissipation problems of the integrated chip.
Thus, at least one automotive manufacturer uses a discrete transient protection circuit to protect the load electronics connected thereto from the above described abnormal conditions. This circuit comprises a 100 joule varistor connected from the battery bus to ground, a high current Schottky diode coupled in series between the positive terminal of the battery and the output of the circuit, and a Zener diode connected in series with a diode clamp between the battery bus and ground. The varistor protects the electronics from positive voltage transients while the Zener diode and diode clamp protect the Schottky diode from destructive reverse voltage transients.
Although the above discrete protection circuit functions quite well, the discrete components compromise space, and performance needs. Additionally, the several discrete components are relatively expensive to the automotive manufacturer.
Hence, there is a need for a simple, reliable and rather inexpensive solid state circuit to replace the discrete protection circuit described above.